Subsea Control Module (SCM)
.png)
The SCM is an independently retrievable unit. SCMs are commonly used to provide well control functions during the production phase of subsea oil and gas production. Typical well control functions and monitoring provided by the SCM are as follows:
- Actuation of fail-safe return production tree actuators and downhole safety valves;
- Actuation of flow control choke valves, shutoff valves, etc.;
- Actuation of manifold diverter valves, shutoff valves, etc.;
- Actuation of chemical injection valves;
- Actuation and monitoring of surface-controlled reservoir analysis and monitoring systems, sliding sleeve, choke valves;
- Monitoring of downhole pressure, temperature, and flow rates;
- Monitoring of sand probes and production tree and manifold pressures, temperatures, and choke positions.
Contents
SCM Components
The typical SCM receives electrical power, communication signals, and hydraulic power supplies from surface control equipment. The subsea control module and production tree are generally located in a remote location relative to the surface control equipment. Redundant supplies of communication signals and electrical and hydraulic power are transmitted through umbilical hoses and cables ranging from 1000 ft to several miles in length, linking surface equipment to subsea equipment. Electronics equipment located inside the SCM conditions the electrical power, processes communications signals, transmits status, and distributes power to solenoid piloting valves, pressure transducers, and temperature transducers.
The subsea electrical module (SEM) is fed with AC power directly from the EPU. It normally incorporates two AC-DC converters and output to DC busbars (typically 24 and 5 V). Actuation of the electro-hydraulic valves is tapped into a 24- busbar, while the sensors are tapped into a 5-V busbar.
Low-flow-rate solenoid piloting valves are typically used to pilot high flow- rate control valves. These control valves transmit hydraulic power to end devices such as subsea production tree valve actuators, choke valves, and downhole safety valves. The status condition of control valves and their end devices is read by pressure transducers located on the output circuit of the control valves.
Auxiliary equipment inside the typical SCM consists of hydraulic accumulators for hydraulic power storage, hydraulic filters for the reduction of fluid particulates, electronics vessels, and a pressure/temperature compensation system. Previous devices have used an oil-filled chamber to compensate for hydrostatic pressure increases outside of the device during use to keep seawater away from cable assemblies.
The SCM is typically provided with a latching mechanism that extends through the body of the SCM and that has retractable and extendable dogs or cams thereon to engage a mating receptacle in a base plate.
SCM Control Mode Description
The SCM utilizes single LP and HP hydraulics, supplied from the umbilical system. Each supply has a “last chance” filter before supply is manifolded to the main supply and pilot supply ports of the solenoid-operated pilot valves.
The LP supply has an accumulator that is mounted internally to the SCM. The HP supply may also have an accumulator. A pressure transducer measures the HP and LP supply pressures for display at the MCS. There are four pilot valves: three on the LP for tree valves and choke, and one on the HP for the downhole safety valve (DHSV), usually referred to the surface-controlled subsurface safety valve (SCSSV). The hydraulic return line has a nonreturn valve to vent used fluid to the sea and prevent seawater ingress.
Valve Actuation
Most of the pilot valves have two solenoids to operate them, one to open and one to close. The solenoids are driven by the solenoid drivers in the SEM.
To open a tree valve, the appropriate solenoid is commanded from the MCS, and the microprocessor in the SEM activates the solenoid driver, which energizes the open solenoid for 2 sec. (typically; this may be adjusted at the MCS). This allows hydraulic fluid to flow into the function line to the tree valve actuator. The pressure in this line will rise very quickly to a value that allows the valve to latch open hydraulically. Thereafter, the valve will remain open as long as the hydraulic supply pressure remains above about 70 bar.
To close a tree valve, the close solenoid is energized in a similar manner to the open solenoid. This causes the spool in the pilot valve to move, venting the hydraulic fluid from the tree valve actuator. The used fluid is vented to sea via a non-return valve.
Choke Operation
The choke has two hydraulic actuators, one to open and one to close, that move the choke via a pawl and ratchet mechanism.
The choke is moved by applying a series of hydraulic pressure pulses to the appropriate actuator. On each pressurize/vent cycle, the choke will move by one step. To provide these pressure pulses, the SCM has two pilot valves, one for opening the choke, and one for closing the choke. These valves are not hydraulically latched and will only pass hydraulic fluid when the solenoid is energized.
To operate the choke, the MCS sends a series of commands to the SCM. For example, if the operator wishes to move the choke by 20 steps, the MCS will send 20 appropriately timed valve commands to the SCM to energize the appropriate solenoid.
References
[1] Society for Underwater Technology, Subsea Production Control, SUT, Subsea Awareness Course, 2008.
[2] B. Laurent, P.S. Jean, L. Robert, First Application of the All-Electric Subsea Production System Implementation of a New Technology, OTC 18819, Offshore Technology Conference, Houston, 2006.
[3] C.P. William, Subsea Control Module, U.S. Patent 6,161,618, 2000.
[4] International Electro-technical Commission, Functional safety of electrical/electronic/programmable electronic safety-related systems, IEC 61508, 2010.
[5] J. Davalath, H.B. Skeels, S. Corneliussen, Current State of the Art in the Design of Subsea HIPPS Systems, OTC 14183, Offshore Technology Conference, Houston, 2002.
[6] International Organization of Standards, Petroleum and natural gas industries - Design and operation of subsea production Systems - Part 6: Subsea production control systems, ISO 13628, 2000.
